Well for standard ATX its a bit harder. You will be limited by not only the motherboard but also GPU.. There are some that are cube shaped such as the Cooler Master HAF XB EVO or the Corsair Carbide Air 540 (I like the design as the PSU is put in the back with the hard drives so the motherboard and GPU have more room and air flow).
https://pcgamingbuilds.com/smallest-atx-case
That's a list of them but always double check that it fits ATX motherboards. The two I listed do fit ATX. Unless you want to swap to a MicroATX ot ITX board you will be limited on how small you can go.